The invention relates to silica reinforced rubber compositions of matter having improved processability, storage stability and or cure for use in tires and mechanical goods.
Particulate fillers such as silica, carbon black, clays, talc, calcium carbonate, silicates (Science and Technology of Rubber, edited by J. E. Mark et al., Academic Press Inc., San Diego, Calif., 1994 p 387-469) and starch (F. G. Corvasce et al., U.S. Pat. No. 5,672,639) have been generally used as reinforcing materials for rubbers to improve their physical properties such as modulus, tensile strength, abrasion, tear properties, and dynamic properties.
The reinforcement of elastomers with particulate fillers such as silica and carbon black has been extensively studied. Four major characteristics of the fillers, namely particle size, morphology, aggregate structure and surface activity, influence the physical and mechanical properties of the reinforced rubber compositions. These characteristics contribute to the reinforcement of the elastomers through interactions between elastomers and fillers, occlusion of the elastomer in the internal voids of the aggregate and agglomeration of the filler aggregates in the elastomer matrix (S. Wolff and M. J. Wang in xe2x80x9cCarbon Black, Science and Technology,xe2x80x9d editors: J. B. Donnet, R. C. Bansal and M. J. Wang, Marcel Dekker, Inc., New York 1993). It is known that several types of interactions exist between molecules which are close to one another, e.g., dispersive, dipole-dipole, induced dipole-dipole, hydrogen bonding and the like. Such interactions can result in different types of cohesive forces. The surface energy of a solid, xcex3s, can be expressed as the sum of several components, each corresponding to a specific type of interaction. For most substances: xcex3s=xcex3sd+xcex3ssp where xcex3sd is the dispersive component of the surface free energy and xcex3ssp (or specific component) is the sum of the other components of the surface free energy. It should be noted that xcex3ssp comprises polar components, e.g., dipole-dipole, hydrogen bonding, and the like.
The difference in the surface free energy of carbon black and silica results in significant differences in the filler-filler and filler-rubber interaction. Compared to carbon black, surface energies of silica with equivalent surface area and structure, have a low dispersive component and a high specific component (M. J. Wang and S. Wolff, Rubber Chemistry and Technology, 65, 329, 1992). The low dispersive component (related to weaker polymer-filler interaction) has been shown to produce low modulus at high strains (S. Wolff, M. J. Wang and E. H. Tan, American Chemical Society, Rubber Division Meeting, Denver, Colo., May 1993). The higher specific component of the surface free energy of silica results in strong filler-filler interaction, resulting in increased viscosity of the rubber composition, especially at low strain rates.
The surface characteristics and hence the surface energy of silica can be changed by surface modification, for example, when the silica surface is chemically modified with so-called coupling systems such as a polyfunctional organosilane, e.g., bis(3-triethoxysilyl propyl)tetrasulfide (TESPT). The specific component of the surface free energy (xcex3ssp) is significantly reduced, leading to improved interaction between silica and rubber for improved compatibility (M. J. Wang, S. Wolff, Rubber Chemistry and Technology, 65, 715, 1992). A reduction of filler-filler interaction results in better dispersion and reduced viscosity. Compared to reinforcing carbon black, silica retards the cure rate of the filled composition. This retardation in cure rate has been attributed to the adsorption of curatives on the silica surface (M. Fetterman, Rubber Chemistry and Technology, 58, 179, 1985).
The silanol content, the adsorbed water content and the surface area of the silica, also affect the cure time (S. Wolff et al., American Chemical Society, Rubber Division Meeting, Denver, Colo., May 1993; M. P. Wagner, Rubber Chemistry and Technology, 49, 703, 1976). Silica, because of its high specific component of surface energy, has a strong tendency to agglomerate and is difficult to disperse in hydrocarbon rubbers. Polyfunctional organosilanes with sulfur linkages such as TESPT improve interactions of filler (e.g., silica) with a polymer, thereby improving the physical properties of vulcanizates such as abrasion resistance and reduced tan xcex4 at 60xc2x0 C. The scorch and cure times are also affected (S. Wolff, M. J. Wang, Tyre Technology Conference, Basel 1993, and Wolff et al., U.S. Pat. No. 4,229,333; Thurn et al., U.S. Pat. No. 3,873,489; Wideman et al., European Patent Application No. EP 0780429A1; R. J. Pickwell, Rubber Chemistry and Technology, 56, 94, 1983; K. J. Sollmann et al., Rubber Division Meeting, American Chemical Society, Cincinnati, Ohio, Fall 1972).
Epoxidized natural rubber (ENR) with up to 50 mole percent epoxidation has been used alone and in combination with other diene rubbers such as natural rubber, styrene butadiene rubber, or butadiene rubber at levels higher than 30 phr (parts per one hundred parts of rubber, on a weight basis) with precipitated silica and mixture of silica with carbon black to improve wet skid resistance, but with poor tire tread abrasion and tear properties. The use of epoxidized natural rubber was accompanied by increased viscosity, retardation of cure rate and poor processability on storage (xe2x80x9cNatural Rubber Science and Technology,xe2x80x9d edited by A. D. Roberts, p.359-456, Oxford University Press, UK, 1988; S. Varughese et al., Kautschuk Gummi und Kunststoffe, 43, 871, 1990). ENR, because of the higher mole percent epoxide groups (10-50%), is reinforced by silica even in the absence of coupling systems such as TESPT or xcex3-mercaptopropyltrimethoxysilane. The addition of a coupling system enhances the cure rate and strength properties of silica-filled ENR (10-50 mole % epoxidation). (M. Nasir et al., European Polymer Journal, 25, 267, 1989; S. Varughese and D. K. Tripathy, Journal of Applied Polymer Science, 44, 1847, 1992). Epoxidized natural rubbers with an epoxy content from 15 to 85 mole percent have been reportedly used in blends with other diene rubbers such as polyisoprene, butadiene, carboxylated nitrile at a 1-15 phr level in silica or silica/carbon black reinforced tread compositions. A synergistic effect of ENR with a carboxylated nitrile was reported to improve vulcanizate properties (Sandstrom, U.S. Pat. No. 5,489,628).
ENR at 5-30 phr was also reportedly used in blends with diene rubber and a coupling system (TESPT) to improve the abrasion and adhesion properties of the vulcanizate. (Segatta et al., U.S. Pat. No. 5,396,940). The addition of glycols, amines or guanidines to rubber compositions containing silica has been reported to counter the retarding effect of silica on the cure rate during vulcanization. Addition of diethylene glycol or triethanolamine in silica-filled rubber reduced the Mooney viscosity and scorch time. The reduction in Mooney viscosity was storage temperature dependant and was not apparently effective at higher storage temperature (M. P. Wagner, Rubber Chemistry and Technology, 49, 703, 1976). Diene rubber compositions with excellent processability and improved dispersibility of silica have been claimed when dicyclohexylamine and diene polymers modified with xe2x80x94COOH, epoxy, amino or hydroxyl group are used in the rubber composition (H. Takamata et al., Japanese Patent No. JP 07292159). Epoxidized soybean and linseed oils have been reported to enhance the adhesion properties of rubber compositions, containing carbon black as major filler and silica as minor filler, with steel cords. No significant effects on viscosity and processability of compositions containing epoxidized soybean and linseed oils were reported with the aforementioned filler blend (Stevens et al., DE 19700967A1, Jul. 16, 1998).
Accordingly, it would be an advantage to provide a process for reducing the cure time and improving the processability of an uncured silica containing rubber composition as well as providing an uncured rubber composition having improved cure time and improved processability. It would also be an advantage to provide a silica containing cured rubber composition produced by the foregoing process, or a silica containing rubber composition having the foregoing properties.
These and other advantages are obtained according to the present invention which comprises a process, product obtained by the process, and a composition that substantially obviates one or more of the limitations and disadvantages of the described prior art processes, products, and compositions.
The present invention comprises a process for manufacturing an uncured silica containing rubber composition having improved processability, improved resistance against the decay of processability during storage of the uncured stock and in some cases reduced cure time by combining the rubber with a class of organic compounds that impart these properties to the rubber composition. The invention also relates to uncured or cured silica containing rubber compositions produced by the process of the invention and uncured and cured silica containing rubber compositions having these organic compounds.
Additional features and advantages of the invention will be set forth in the written description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the process, product-by-process and composition particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, the invention in one aspect comprises a process for improving the processability, storage stability and in some cases cure rate of silica reinforced rubber, compositions comprising the step of incorporating into said silica and rubber appropriate amounts (preferably less than about 10 phr) of at least one organic compound, such as an organic compound comprising a relatively low molecular weight organic compound (i.e., a molecular weight of from about 50 to less than about 7000, and especially from about 100 to about 2000) containing at least one functional group (preferably not intramolecularly reactive) and located in terminal or sterically unhindered positions on the molecule, selected from an epoxy functional group, such as an epoxy/ether, epoxy/hydroxyl, epoxy/ester, epoxy/amine, ether/amine, cycloaliphatic ether/hydroxyl, episulfide, episulfide/amine, episulfide/ether, episulfide/hydroxyl or episulfide/ester group. These compounds contain aliphatic and/or cycloaliphatic groups. Aliphatic groups comprise both saturated and unsaturated branched chain or straight chain alkyl groups, whereas cycloaliphatic groups include both saturated and unsaturated ring structures based on carbon, or heterocyclic ring structures based on carbon that contain, in addition to carbon in the ring, oxygen, or sulfur, or nitrogen.
The organic compound also comprises abietyl compounds and especially abietyl amine compounds, styrenated resorcinol formaldehyde epoxy polymer compounds, or ester hydroxy organic compounds that contain at least one hydroxyl and especially at least about two hydroxyl groups and at least one ester or one ether group, and combinations of any of the foregoing organic compounds.
In its broadest aspect, the process of the invention comprises incorporating the low molecular weight organic compound into the silica reinforced rubber by combining a mixture comprising rubber, silica and at least one organic compound as described herein in any sequence, i.e., it involves not only the addition of the organic compound to rubber already containing silica but also combining rubber, silica and the organic compound in any sequence or simultaneously.
The invention also comprises a product made by the aforesaid process including both uncured and cured rubber compositions and a composition of matter containing such organic compounds in a silica rubber composition, including both uncured and cured compositions of matter.
In another embodiment, the invention comprises uncured and cured rubber compositions used in tire, curing bladder and mechanical goods applications, and reinforced with silica or silica in combination with minor quantities of other particulate fillers such as carbon black, clay, silicates, and starch with xe2x80x94OH, xe2x80x94Oxe2x80x94, or ester functionalities.
In a further embodiment, the invention comprises rubber compositions combined with certain specific abietyl, styrenated resorcinol formaldehyde, and ester hydroxy organic compounds that contain at least one hydroxyl and especially at least about two hydroxyl groups and at least one ester or ether group, found to improve the processability and storage stability of uncured silica reinforced rubber compositions. These ester hydroxy compounds generally comprise ester diols where the ester, ether and hyroxy groups comprise functional groups. The addition of these organic compounds surprisingly lowers the viscosity and also reduces the rate of increase of the viscosity of the uncured rubber composition as a function of storage time, compared to a control composition with none of these organic compounds.
The invention also comprises silica containing tire compositions such as those used in tread, tread cushion, sidewalls, carcass, belt, overlay, liner cushion, innerliner and bead area construction made from the aforesaid rubber compositions. In another aspect the invention relates to the use of the aforementioned organic compounds, especially epoxy compounds that reduce the cure time and increase the processability of rubber compositions.
The uncured rubber compositions of the invention containing silica or silica in combination With minor quantities of other particulate fillers, e.g., carbon black, clay, and the like, for use in tire compositions, have improved processability, storage stability and in some cases cure time. The organic compounds of the invention provide these enhancements in processing and/or cure properties.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the written description serve to explain the principles of the invention.